1. Field of the Invention
The present invention relates to a modulator and demodulator (MODEM) communication system. More particularly, it relates to a multipoint type MODEM communication system having training means for adapting a receiving MODEM to a characteristic of a MODEM communication line, and a method for training the same.
2. Description of the Related Art
MODEM communication systems using telephone lines as communication lines are extensively known. The MODEM communication systems are categorized into two types; a point-to-point system in which a pair of MODEMs are connected through the telephone line, and a multipoint system in which a MODEM is connected to a plurality of MODEMs through a common telephone line. The point-to-point system has the advantages of a simple circuit construction for the MODEM and a simple communication protocol between the MODEMs. However, if a system is required in which a large number of MODEMs are provided and connected to each other, the point-to-point system has the disadvantages of many lines in use, with corresponding high line-use fees, etc. The multipoint system solves these problems because a single telephone line is commonly used for the plurality of MODEMs, reducing the line-use fees and the construction costs. The present invention essentially relates to the multipoint MODEM communication system.
In the multipoint MODEM communication system, however, a process known in the art as "TRAINING", which will be described later in detail, is indispensable and must be carried out prior to the reception of data on the line. Briefly, the training process is that wherein a receiving MODEM adjusts control parameters therein in response to a training signal from a sending MODEM to ensure the reception of data. In the multipoint MODEM communication system, distances between a center station (CS) and the plurality of local stations (LS) are not constant, and accordingly, a characteristic of the line, for example, phase jitter, between the CS and one LS is not equal to that of a line between the CS and another LS, and thus, for example, the receiving MODEM in the CS, per se, must adjust reception parameters therein to match the line connected to the sending MODEM in the LS, prior to the reception of data from the sending MODEM.
In general, a high speed transfer on the telephone line may result in a short transfer time for the same amount of data transfer. In the multipoint MODEM communication system, however, this is not always true because a considerable length of time is required for the training. Namely, the high speed transfer will require, for example, a fine adjustment of the parameters in the receiving MODEM, and this fine adjustment may require a large amount of training information and the training may take a long time. For example, according to CCITT recommendations, the training time should be within 50 ms for a 4800 bit per second (bps) transfer line (CCITT V. 27 bis), 253 ms for a 9600 bps transfer line (CCITT V. 29), and 1393 ms for 14.4 Kbps transfer line (CCITT U. 33). If only a small amount of data is to be transferred, the time needed for the training may be longer than the time needed for the data transfer. As a result, the transfer time containing the training time and the data transfer time in the high speed transfer line may be longer than the transfer time in the low speed transfer line. To eliminate this paradox, the training time must be reduced. Shortening the training time will bring a high transfer efficiency on the line, and accordingly, reduce the line-use fees.
A variety of training methods for shortening the training time have been proposed. The prior art training methods, however, still suffer from the disadvantage of a long training time.